1. Field
Disclosed herein is a copper alloy which does not contain an element which has an adverse environmental effect such as Be, and a process for producing the same. This copper alloy is suitable for electrical and electronic parts, safety tools, and the like.
2. Description of Related Art
Examples of the electric and electronic parts include connectors for personal computers, semiconductor plugs, optical pickups, coaxial connectors, IC checker pins and the like in the electronics field; cellular phone parts (connector, battery terminal, antenna part), submarine relay casings, exchanger connectors and the like in the communication field; and various electric parts such as relays, various switches, micromotors, diaphragms, and various terminals in the automotive field; medical connectors, industrial connectors and the like in the medical and analytical instrument field; and air conditioners, home appliance relays, game machine optical pickups, card media connectors and the like in the electric home appliance field.
Examples of the safety tools include excavating rods and tools such as spanner, chain block, hammer, driver, cutting pliers, and nippers, which are used where a possible spark explosion hazard may take place, for example, in an ammunition chamber, a coal mine, or the like.
A Cu—Be alloy, known as a copper alloy is used for the above-mentioned electric and electronic parts. This alloy is strengthened by age precipitation of the Be, and contains a substantial amount of Be. This alloy has been extensively used as a spring material or the like because it is excellent in both tensile strength and electric conductivity. However, Be oxide is generated in the production process of Cu—Be alloy and also in the process of forming to various parts.
Be is an environmentally harmful material as is Pd and Cd. Particularly, intermetallics of a substantial amount of Be in the conventional Cu—Be alloy necessitates a treatment process for the Be oxide in the production and working of the copper alloy because it leads to an increase in the production cost. It also causes a problem in the recycling process of the electric and electronic parts because the Cu—Be alloy is a problematic material from the environmental point of view. Therefore, the emergence of a material, excellent in both tensile strength and electric conductivity, without containing environmentally harmful elements such as Be is desired.
It is very difficult to simultaneously enhance both the tensile strength [TS (MPa)] and the electric conductivity [relative value of annealed copper polycrystalline material to conductivity, IACS (%)]. Therefore, the end user frequently requests a concentrate with either of these characteristics. This is also shown in Non-Patent Literature 1 describing various characteristics of practically produced copper and brass products.
FIG. 1 shows the relation between tensile strength and electric conductivity of copper alloys free from harmful elements such as Be described in Non-Patent Literature 1. As shown in FIG. 1, in conventional copper alloys free from harmful elements such as Be, for example, the tensile strength is as low as about 250-650 MPa in an area with a electric conductivity of 60% or more, and the electric conductivity is as low as less than 20% in an area with a tensile strength of 700 MPa or more. Most of the conventional copper alloys are high in either tensile strength (MPa) or the electric conductivity (%). Further, there is no high-strength alloy with a tensile strength of 1 GPa or more.
For example, a copper alloy called Corson alloy, in which Ni2Si is precipitated, is proposed in Patent Literature 1. This alloy has a relatively good balance of tensile strength and electric conductivity among alloys free from environmentally harmful elements such as Be, and has a electric conductivity of about 40% at a tensile strength of 750-820 MPa.
However, this alloy has limitations in enhancing strength and electric conductivity, and this still leaves a problem from the point of product variations as described below. This alloy has age hardenability due to the precipitation of Ni2Si. If the electric conductivity is enhanced by reducing the contents of Ni and Si, the tensile strength is significantly reduced. On the other hand, even if the contents of Ni and Si are increased in order to raise the precipitation quantity of Ni2Si, the electric conductivity is seriously reduced since the rise of tensile strength is limited. Therefore, the balance between tensile strength and electric conductivity of the Corson alloys is disrupted in an area with high tensile strength and in an area with high electric conductivity, consequently narrowing the product variations. This is explained as follows.
The electric resistance (or electric conductivity that is the inverse thereof) of this alloy is determined by electron scattering, and fluctuates depending on the kinds of elements dissolved in the alloy. Since the Ni dissolved in the alloy noticeably raises the electric resistance value (noticeably reduces the electric conductivity), the electric conductivity reduces in the above-mentioned Corson alloy if Ni is increased. On the other hand, the tensile strength of the copper alloy is obtained due to an age hardening effect. The tensile strength is improved more as the quantity of precipitates grows larger, or as the precipitates are dispersed more finely. The Corson alloy has limitations in enhancing the strength from the point of the precipitation quantity and from the point of the dispersing state, since the precipitated particle is made up of Ni2Si only.
Patent Literature 2 discloses a copper alloy with a satisfactory wire bonding property, which contains elements such as Cr and Zr and has a regulated surface hardness and surface roughness. As described in an embodiment thereof, this alloy is produced based on hot rolling and solution treatment.
However, the hot rolling needs a surface treatment for preventing hot cracking or removing scales, which result in a reduction in yield. Further, frequent heating in the atmosphere facilitates oxidation of active additive elements such as Si, Mg and Al. Therefore, the generated coarse internal oxides problematically s cause deterioration of characteristics of the final product. Further, the hot rolling and solution treatment need an enormous amount of energy. The copper alloy described in the cited literature 2 thus has problems in view of an addition in production cost and energy saving, furthermore, deterioration of product characteristics (bending workability, fatigue characteristic and the like besides tensile strength and electric conductivity), which is result of generation of coarse oxides and the like, because this alloy is based on the hot working and solution treatment.
FIGS. 2, 3 and 4 are a Ti—Cr binary system state view, a Cr—Zr binary system state view and a Zr—Ti binary system state view, respectively. It is apparent from these figures, the Ti—Cr, Cr—Zr or Zr—Ti compounds tend to formed, in a high temperature range after solidification in a copper alloy containing Ti, Cr or Zr. These compounds inhibit fine precipitation of Cu4Ti, Cu9Zr2, ZrCr2, metal Cr or metal Zr which is effective for precipitation strengthening. In other words, only a material insufficiently strengthened by precipitation with poor ductility or toughness can be obtained from a copper alloy produced through a hot process such as hot rolling. This also shows that the copper alloy described in Patent Literature 2 has a problem in the product characteristics.
On the other hand, the safety tool materials have required mechanical properties, for example, strength and wear resistance matching those of tool steel. It is also required to avoid generating sparks which could cause an explosion i.e. excellent spark generation resistance is necessary. Therefore, a copper alloy with high thermal conductivity, particularly, a Cu—Be alloy aimed at strengthening by age precipitation of Be has been extensively used. Although the Cu—Be alloy is an environmentally problematic material, as described above, it has been heavily used as the safety tool material based on the following.
FIG. 5 is a view showing the relation between electric conductivity [IACS (%)] and thermal conductivity [TC (W/m·K)] of a copper alloy. As shown in FIG. 5, both are almost in a 1:1-relation, which enhances the electric conductivity [IACS (%)] which is the same as enhancing the thermal conductivity [TC (W/m·K)], in other words, it enhances the spark generation resistance. Sparks are generated by the application of a sudden force by an impact blow or the like during the use of a tool due to a specified component in the alloy being burnt by the heat generated by an impact or the like. As described in Non-Patent Literature 2, steel tends to cause a local temperature rise due to its thermal conductivity which can be as low as ⅕ or less of that of Cu. Since the steel contains C, a reaction “C+O2→CO2” takes place, generating sparks. In fact, it is known that pure iron containing no C generates no sparks. Other metals which tend to generate sparks are Ti and Ti alloy. The thermal conductivity of Ti is as extremely low, as low as 1/20 of that of Cu, and therefore the reaction “Ti+O2 to TiO2” takes place. Data shown in Non-Patent Literature 1 are summarized in FIG. 5.
However, the electric conductivity [IACS (%)] and the tensile strength [TS (MPa)] are in a trade-off relation, and it is extremely difficult to enhance both simultaneously. Therefore, the Cu—Be alloy was the only copper alloy that had sufficiently high thermal conductivity TC while retaining a tool steel-level high tensile strength in the past.    Patent Literature 1:
Japanese Patent No. 2572042    Patent Literature 2:
Japanese Patent No. 2714561    Non-Patent Literature 1:
Copper and Copper Alloy Product Data Book, Aug. 1, 1997, issued by Japan Copper and Brass Association, pp. 328-355    Non-Patent Literature 2:
Industrial Heating, Vol. 36, No. 3 (1999), Japan Industrial Furnace Manufacturers Association, p. 59